In order to impart higher conductivity to π-conjugated polymers, doping of the π-conjugated polymers with dopants have been carried out.
By the way, polymers in which π-conjugation has been developed inherently have high flatness of polymer chain and have high crystallizability (stacking property) between polymer chains due to affinity of π-bonds. When such a π-conjugated polymer is doped with a dopant, the above flatness and crystallizability are more enhanced. On this account, solubility (solubility due to heat or solvent) of the π-conjugated polymer doped with a dopant is usually lowered.
Therefore, it is known that reconciliation of high electrical conductivity and high solubility (solubility due to heat or solvent) with each other in the π-conjugated polymer is difficult.
In this connection, an attempt to prepare conductive polyaniline improved in solubility has been made in Japaneses Patent No. 4,035,353 (“JP-‘353”). More specifically, a monomer of polyaniline is reacted with a surfactant agent comprising a metal sulfonate, ammonium sulfonate or phosphoric acid ester having a repeating unit of alkylene ether in its molecular structure in an aqueous solution to form an aniline-surfactant agent salt of an amphipathic structure, and this salt is subjected to oxidation polymerization as a monomer. Subsequently, to an aqueous solution of the conductive polyaniline obtained, a ketone-based solvent or an aromatic solvent is added to separate a supernatant liquid, and in the supernatant liquid, the conductive polyaniline and the solvent are compatibilized with each other to prepare a conductive polyaniline solution for forming a conductive coating film.
In the conductive polyaniline solution prepared in the above mentioned JP-'353, however, there is a problem that particulate conductive polyaniline remains and is not homogeneously dissolved. Further, there is another problem that when a film is formed from the solution containing the particulate conductive polyaniline, unevenness was observed.
A π-conjugated polymer doped with a dopant is favorably used as a component for forming an electrolyte polymer layer of a dye-sensitized solar cell. The dye-sensitized solar cell usually has a laminated structure in which a transparent substrate, a light-transmitting electrode, a metal oxide layer having a dye supported thereon, an electrolyte polymer layer, a counter electrode and a counter electrode substrate are laminated in this order (see, for example, International Publication No. 013942/2009 Pamphlet). The electrolyte polymer layer contains a π-conjugated polymer doped with a dopant and an ionic compound having an ionic species replaceable with a dopant ionic species of the dopant. In the course of generation of electromotive force by the dye-sensitized solar cell, the above species are replaced with each other in the electrolyte polymer layer to transfer charge. For actually forming the electrolyte polymer layer, the π-conjugated polymer doped with a dopant and the ionic compound are blended with a solvent to prepare a solution. However, if the conductive polyaniline solution prepared in the aforementioned JP-'353 is used and if an ionic compound is blended, there arises a problem that the conductive polyaniline is aggregated and precipitated. In this case, a film of unevenness is only formed, and such a film cannot constitute a dye-sensitized solar cell.
Also in a conductive polymer composition in which a dopant and a π-conjugated polymer are contained but the dopant is not incorporated into the π-conjugated polymer, there are problems of such solubility as above and stability of the resulting solution.
Accordingly, it is an object of the present invention to provide a conductive polymer composition which can be homogeneously dissolved in a solvent and can form a smooth film. Moreover, it is an object of the present invention to provide a conductive polymer composition which is free from aggregation even if an ionic compound is added to a solvent together with this conductive polymer composition, and can form a smooth film. In other words, it is an object of the present invention to provide a conductive polymer composition capable of forming a solution of high stability.